6.7.4 Fossil Fuel Organic Carbon Aerosol

Anthropogenic organic aerosols are a by-product of fossil fuel and biomass
combustion and they consist of many complex chemical compounds and are released
either as primary aerosol particles or as volatile organic gases (see Chapter
5). Studies that investigate the radiative forcing due to organic carbon
(OC) from fossil fuels are included in Table 6.5. Penner et al. (1998b) and
Grant et al. (1999) found a DRF of +0.16 Wm-2 when modelling the
direct radiative forcing due to an internal mixture of fossil fuel BC and OC,
and +0.2Wm-2 when modelling the radiative forcing due to externally
mixed fossil fuel BC. From these results, an annual global mean radiative forcing
of -0.04 Wm-2 for fossil fuel OC from a global mean burden of approximately
0.7 mgm-2 may be derived. However, if OC were modelled as an external
mixture with BC and/or if the effects of relative humidity are included, the
radiative forcing due to OC from fossil fuels is likely to be more negative,
thus this represents an approximate weakest limit. An alternative method for
calculating the DRF due to fossil fuel OC from the results of Penner et al.
(1998b) is to note that the absorption is approximately doubled when BC is modelled
as an internal mixture rather than an external mixture (e.g., Haywood et al.,
1997a). Thus, for an external mixture of fossil fuel OC a radiative forcing
of -0.24 Wm-2 may be more appropriate. Cooke et al. (1999) performed
GCM calculations for externally mixed fossil fuel OC, finding a radiative forcing
of -0.02 Wm-2 from a global mean burden of 0.34 mgm-2.
Myhre et al. (2001) scale the atmospheric concentrations of fossil fuel OC to
modelled sulphate aerosol concentrations and include the effects of relative
humidity to estimate a radiative forcing of -0.09 Wm-2 from a global
mean burden of 0.66 mgm-2. Thus, modelling estimates suggest that
the normalised radiative forcing for OC is in the range -60 to -340 Wg-1,
which is smaller in magnitude than that due to BC due to the larger specific
extinction coefficient for BC and the fact that BC may exert a significant radiative
forcing in cloudy regions. Cooke et al. (1999) assume that OC is partially absorbing
with a modelled single scattering albedo of approximately 0.97 at a wavelength
of 0.55 µm. Hansen et al. (1998) use a three-dimensional GCM and the OC
distribution from Liousse et al. (1996) and estimate the radiative forcing due
to combined fossil fuel and biomass sources to be -0.41 Wm-2. The
approximate fraction of the atmospheric burden of the fossil fuel component
may be estimated from the emission inventory of Liousse et al. (1996) who estimate
that fossil fuels contribute 38% to the total OC emissions. Thus the radiative
forcing due to fossil fuel OC may be inferred to be approximately -0.16 Wm-2.
This may constitute an approximate upper estimate as the majority of fossil
fuel OC occurs over mid-latitude land areas where the surface reflectance is
generally higher and insolation is lower than in the equatorial regions where
biomass burning is the major source of OC. From these calculations, the radiative
forcing due to fossil fuel OC is estimated to be -0.10 Wm-2. The
uncertainty associated with this estimate is necessarily high due to the limited
number of detailed studies and is estimated to be at least a factor of three.